The present invention relates generally to a process for manufacturing turbine airfoil or blade components used in gas turbine engines. More specifically, the present invention relates to a process for manufacturing turbine airfoils or blades to limit the rubbing of any coatings that are applied to tips of the turbine airfoils.
Turbine blade materials, typically superalloys, achieve the desired high temperature properties through directional or single crystal solidification as well as by composition. However, one of the consequences of certain alloying elements in composition is inadequate environmental resistance in a gas turbine operating environment.
Components such as turbine blades, operating in the gas path environment of gas turbine engines are subjected to significant temperature extremes and degradation by the oxidizing and corrosive environments caused by the hot gases of combustion. Protective coating systems, such as environmental coatings and thermal barrier coating (TBC) systems, are often applied to the external surfaces of these components to protect the bare alloy from this operating environment. In the case of TBC systems, the TBC system also affords the opportunity to improve the efficiency of the engine by permitting an increase in the operating temperatures of the engine. The environmental coating systems are generally comprised of a metallic environmental coating that serves as a bond coat applied to the structural component. When used in a TBC system, an insulating ceramic layer is applied over the environmental coating. Common bond coats or environmental coatings for turbine airfoils and combustor components can be classified into two types, overlay coatings and diffusion aluminide coatings.
Overlay coatings such as MCrAlY type or NiAl based coatings can be applied by physical vapor deposition (PVD) processes such as sputtering, cathodic arc, electron beam, etc., or by plasma spray processes. Coating composition, microstructure and thickness are controlled by the processing parameters. Diffusion aluminide coatings are widely employed in the industry, and can be applied by a range of methods including pack cementation, above-the-pack processing, vapor phase processing, chemical vapor deposition and slurry coating processes. The thickness and aluminum content of the end product coating can be controlled by varying the coating time, coating temperature, the aluminum activity of the coating process and subsequent heat treatments. Often, these coatings are enhanced by including noble metals and/or reactive elements within the coating. To complete the TBC system, the ceramic top coat, typically a yttria stabilized zirconia (YSZ), for rotating turbine airfoils is preferably deposited by electron-beam PVD, although plasma spray processes are widely used for stationary vane and combustor applications. Improvements to TBC systems are constantly under development to permit the airfoil system to which they are applied reach higher operating temperatures or to have a longer operational life at current temperatures.
These coating systems are often applied to fully machined blades that are then assembled into the turbine disks, etc. and then into the final engine assembly. Although machining operations are precise for each component, within prescribed manufacturing tolerances, the stack-up of these manufacturing tolerances in the assembled components of the final engine assembly can result in significant variations. Because of these variations, the tips of the blades often severely rub mating shroud materials during the initial cycles of engine operation. This rubbing causes the removal of the environmental coating and TBC system from the tip areas of the blades, allowing direct exposure of the underlying less oxidation resistant superalloy to hot oxidizing or corrosive gases. The consequence of the removal of the environmental coating from the tip area is a more rapid oxidation of the superalloy causing tip recession. An increase in the clearance between the blade tip and the shroud can also result from these variations and is equally undesirable, as these clearances cause significant loss in engine efficiency and increase engine operating temperatures, further exacerbating the problem. Although the coatings originally applied to these tip areas are designed to withstand the hot oxidizing atmosphere, they are frequently removed because of the imprecise control of the clearances. The removal or partial removal of advanced TBC systems preclude taking full advantage of the improvements afforded by the advanced TBC systems.
U.S. Pat. No. 5,191,711 to Vickers et al. discusses a fixture for placing and holding of compressor blades or turbine blades in their normal xe2x80x9crunning position.xe2x80x9d After the blades are placed in their xe2x80x9crunning position,xe2x80x9d the tips of the blades can be machined to achieve a uniform tip clearance. Environmental coatings and thermal barrier coating systems are not normally applied to compressor hardware. Thus, it is not surprising that Vickers does not discuss TBC or environmental coatings on the blade tips in general and does not specifically discuss how to maintain the TBC or environmental coatings on the blade tip during operation.
Therefore, what is needed in the art is a process for sequencing the assembly process for turbine blades or airfoils into the turbine engine to limit the extent of tip rubbing and environmental coating removal that causes deterioration of tips of TBC coated turbine engine airfoils during engine operation, while also assuring that there is minimal clearance between the blades and shrouds that could adversely affect engine efficiency.
The present invention discloses several related techniques for processing turbine airfoils or blades to achieve optimal tip performance and reduce the wear or rubbing of thermal barrier coating (TBC) or environmental coatings from the blade tip of the turbine airfoil.
A first method of processing turbine blades to limit removal of applied coatings from tips of the turbine blades includes assembling a plurality of turbine blades into a turbine disk. The blades are dimensionally measured and a set of turbine blades not having appropriate fit-up dimensions is determined. The set of turbine blades not having appropriate fit-up dimensions is then machined to achieve a predetermined fit-up dimension. Afterwards, protective coatings (environmental coatings and TBC systems) are applied to the turbine blades.
A second method of processing turbine airfoils includes assembling a plurality of turbine airfoils onto a turbine disk. Each turbine airfoil assembled on the turbine disk is then measured. The turbine airfoils are then processed as required, using the measurement of each turbine airfoil, to have a length less than that required for appropriate fit-up dimensions. Finally, protective coatings (environmental coatings and TBC systems) are applied to the turbine airfoils in amounts sufficient to achieve the appropriate fit-up dimensions for each turbine airfoil.
A third method of processing turbine blades for appropriate fit-up dimensions includes applying protective coatings to a plurality of turbine blades. The coated turbine blades are then assembled onto a turbine disk. The turbine disk is then assembled into a turbine engine and the turbine engine is run with the blades assembled on the turbine disk. Alternatively, a set of coated turbine blades not having appropriate fit-up dimensions is then determined. The set of coated turbine blades not having appropriate fit-up dimensions have material removed to achieve appropriate fit-up dimensions. Finally, a slurry or other coating is applied to the turbine blade to restore any protective coatings that may have been removed.
The present invention can significantly improve the performance of blade tips by assuring not only proper fit-up of the blade to rotor but also that appropriate coatings are on the proper fitting blade. Consequently, this improvement in blade tip performance can extend the service intervals for engines operating with these blade tips because a high percentage of shop visits are dictated by performance loss and increase in engine gas temperatures due to tip recession that occurs after protective coatings are lost due to early engine excursions. In addition, this improvement in blade tip performance can extend the time period of adequate engine efficiency and improve the repairability of airfoils with TBC bond coats.
One advantage of the present invention is that it provides extended performance capabilities for engines using environmental coatings on turbine airfoils. Another advantage of the present invention is that it provides improved repairability of airfoils following engine use. Still another advantage of the present invention is that existing processes can be used to achieve these improvements. A further advantage of the present invention is that the entire benefit resulting from advanced TBC systems can be utilized after initial engine cycling and break-in. An additional advantage of the present invention is that improved control of fit-up results in improved engine performance from less shroud abrasion and a better seal between the blade tip and the shroud.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.